Public cellular networks (public land mobile networks) are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as designated IS-54B in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
Traditional analog radiotelephone systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well-known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. These discrete frequency bands serve as channels over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands, a system designated EIA-553 or IS-19B. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 Mhz. At present there are 832, 30-Khz wide, radio channels allocated to cellular mobile communications in the United States.
The limitations on the number of available frequency bands present several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system generally requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of frequency bands available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system of only several hundred frequency bands.
Another technique which may further increase channel capacity and spectral efficiency is time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots. Although communication on frequency bands typically occur on a common TDMA frame that includes a plurality of time slots, communications on each frequency band may occur according to a unique TDMA frame, with time slots unique to that band. Examples of systems employing TDMA are the dual analog/digital IS-54B standard employed in the United States, in which each of the original frequency bands of EIA-553 is subdivided into 3 time slots, and the European GSM standard, which divides each of its frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during the user's assigned time slots.
A channel in a TDMA system typically includes one or more time slots on one or more frequency bands. As discussed above, traffic channels are used to communicate voice, data or other information between users, for example, between a mobile terminal such as a radiotelephone and a network base station. In this manner, each traffic channel forms one direction of the duplex communications link established by the system from one user to another. Traffic channels typically are dynamically assigned by the system when and where needed. In addition, systems such as the European GSM system, may "frequency hop" traffic channels, i.e., randomly switch the frequency band on which a particular traffic channel is transmitted. Frequency hopping reduces the probability of interference events between channels, using interferer diversity and averaging to increase overall communications quality.
Typically included in the dedicated control channels transmitted in a cell are forward control channels which are used to broadcast control information in a cell of the wide area cellular network to radiotelephones which may seek to access the network. The control information broadcast on a forward control channel may include such things as the cell's identification, an associated network identification, system timing information and other information needed to access the wide area cellular network from a radiotelephone.
Forward control channels, such as the Broadcast Control Channel (BCCH) of the GSM standard, typically are transmitted on a dedicated frequency band in each cell. A radiotelephone seeking access to a system generally "listens" to a control channel in standby mode, and is unsynchronized to a base station or satellite until it captures a base station or satellite control channel. In order to prevent undue interference between control channels in neighboring cells, frequency reuse is conventionally employed, with different dedicated frequency bands being used for the control channel in neighboring cells, according to a frequency reuse pattern that guarantees a minimum separation between cochannel cells. Frequency hopping, which might allow denser reuse of control channel frequency bands, is typically not employed because an unsynchronized radiotelephone generally would have difficulty capturing a frequency-hopped control channel due to lack of a reference point for the frequency hopping sequence employed. Moreover, for private uncoordinated radio communications systems, a frequency reuse pattern cannot be used because each private radio communications system typically operates independently of other potentially interfering systems including the wide area cellular network.
The past decades have shown a considerable rise in the deployment of mobile telephony. With a slow start of the analog standards like AMPS, NMT and TACS, mobile telephony has really hit the consumer markets with the advanced digital standards incorporating TDMA like GSM and D-AMPS. In addition to progress in mobile terminal features like size and battery life, much progress has been made at the network side as well. Increasingly dense cell reuse plans have been complemented with hierarchical cell structures, where macrocells cover entire districts, microcells cover smaller parts like streets, and picocells cover very small areas the size of a few rooms. Important for the hierarchical cell structure is that each base station deployed (ranging from macro to pico base stations) is part of the same Public Land Mobile Network (PLMN), also referred to as a wide area cellular network. When a mobile user wanders from a macrocell to a microcell area, the call can be handed off from a macro base station to a micro base station without the user noticing it. This is particularly true for digital phone systems that apply TDMA: being handed off from one base station to another within a coordinated wide area cellular network for the mobile phone usually only involves the change of a time slot.
Recently, private radio communications networks for residential and business areas are being developed that use the same air-interface as the public cellular network, but do not form an integrated part with the overlaying public cellular network. In this sense, these private systems are not micro or pico networks since there is no direct connection between these private systems and the public cellular network. For example, for residential usage, private base stations can be used as described in U.S. Pat. No. 5,428,668 which only connect to the PSTN (or ISDN) wired network. In business or office networks applying a Private Branch Exchange ("PBX"), radio base stations belonging to the same private network communicate with each other, but none of them communicates directly with the overlaying wide area cellular network. A mobile terminal in range of a private communications network typically preferably enters a private mode attaching to the private communications network to enjoy benefits such as lower tariffs, longer talk and stand-by time, and better voice quality.
However, in such systems, the mobile terminal, once it accesses the private communications network, typically cannot receive incoming calls from the wide area cellular network. While it has been suggested to resolve this problem by providing for call forwarding in the wide area cellular network to the PSTN number of the private radio communications network when the mobile terminal is connected to the private radio communications network, this is not always desirable. For example, there may be occasions where call forwarding from the wide area cellular network number to the PSTN number of the private radio communications network is not available. Furthermore, some wide area cellular networks employ a short message service (SMS) which is not defined on wired networks and, accordingly, such short messages are not presently forwarded through call forwarding.